This rewritten, focused proposal with new preliminary data is in response to the last two IRG reviews. The previous funding period identified osteochondral progenitor cells, referred to as mesenchymal stem cells (MSC), isolated from bone marrow and/or periosteum of many animal species and humans which, when incorporated into a type l collagen carrier, provided a composite that successfully regenerated full and partial thickness articular cartilage defects of young adult rabbits. These preliminary findings are the basis of the central hypothesis of this proposal, i.e., that the functional repair of adult hyaline cartilage is controlled by the entrance and chondrogenic lineage progression of uncommitted MSCs under the direction of specific biological and mechanical influences, and represents a recapitulation of embryonic events. The long-term goal of these investigations is to optimize the functional regeneration/repair of adult articular cartilage and to establish clinically relevant protocols to enhance this complex process. The functional regeneration/repair of full and partial thickness articular cartilage weight-bearing defects of rabbits will be optimized by pretreatment of defects with a dilute trypsin solution to prepare the site for implantation. The repair will be assessed by morphologic, immunohistochemical and biomechanical criteria. The repair of defects with MSCs will be compared to that with committed chondrocytes and whole marrow clot. The cellular and molecular sequence and control mechanisms of the optimized repair/regeneration will be detailed by unique vector- marking experiments, vital dye cell staining, molecular biologic, biochemical and autoradiographic studies. In situ hybridization will be an important bridge between the morphologic analysis and molecular biology. These studies are intended to provide the basis of a better understanding of the controlling mechanisms operative in determining lineage entrance and progression of MSCs into chondrocytes. Finally, the long-term repair/regeneration of these articular cartilage defects using optimized MSC methodologies will be studied in young adult and "aged" rabbits. These focused investigations are designed to establish specific protocols for clinically relevant regeneration/repair of adult articular cartilage.